Two Superior Quality Bits at One Value Price Aanraku's Twofers Grinder Bits offer superior quality at a low-cost. The diamond abrasive grinding surface is double plated for strength, durability and long life. It's large 18mm bit height gives you a larger grinding surface, designed to maximize bite and allows for the uniform grinding. Diamond coating guaranteed not to peel! Ultra fine bit is ideal for delicate glass, such as dichroic, mirror and antique, to prevent chipping. Also an excellent edge finisher.